In a conventional MIS semiconductor having a metal-insulator-silicon structure, the gate electrode and wirings are made of polycrystal silicon. In modern highly integrated circuits, however, polycrystal silicon poses a serious problem of propagation delay. To solve this problem, the gate electrode and wirings are made of a high melting point metal such as molybdenum (Mo) and tungsten (W) which has a lower resistance than polycrystal silicon. FIG. 2(a) is a schematic cross sectional view of an MIS semiconductor device in which the gate electrode 13 composed of high melting point metal, and source and drain regions 15 are formed. The reference numeral 14 in FIG.2.(a) denotes damaged insulation (SiO.sub.2) in a layer near the gate electrode 13. This damage is caused by the etching of the high melting point metal 13 or the ion implantation to form the source and the drain regions 15. In order to recover the damage in the insulation layer 14, the semiconductor device undergoes heat treatment in an oxidation atmosphere for re-oxidation. During the heat treatment, a film 17 of molybdenum oxide or tungsten oxide is formed as shown in FIG.2(b). It is unstable and fluctuates the characteristic properties of the completed element. In addition, it is sublimatable and consequently the heat treatment should be carried out under strictly controlled conditions. For this reason, the yield heretofore has been very low for semiconductor elements produced according to the process shown in FIG.2.